The electrophotography process used in some imaging devices, such as laser printers and copiers, utilizes electrical potentials between components to control the transfer and placement of toner. These electrical potentials create attractive and repulsive forces that tend to promote the transfer of charged toner to desired areas while ideally preventing transfer of the toner to unwanted areas. For instance, during the process of developing a latent image on a photoconductive surface, negatively charged toner particles may be deposited onto more positively charged latent image features (e.g., corresponding to text or graphics) on the photoconductive surface. At the same time, the negatively charged toner particles may be prevented from transferring or migrating to more negatively charged areas (e.g., corresponding to the document background) of the same photoconductive surface. In this manner, imaging devices implementing this process may simultaneously generate images with fine detail while maintaining clean backgrounds.
The precise magnitudes of these electrical potentials and the nature of the voltages (e.g., AC or DC) varies among devices and manufacturers. In general, however, a laser or optical imaging source is used to illuminate and selectively discharge portions of a photoconductive surface to create a latent image having a lower surface potential than the remaining, undischarged areas of the photoconductive surface. The toner is charged to some intermediate level between the discharge potential of the latent image and the surface potential of the undischarged photoconductive surface. The toner may be charged triboelectrically and/or via biased toner delivery control components, such as a toner adder roll, a doctor blade, and a developer roller. The developer roller supplies toner to develop the latent images on the photoconductive surface. The developed image is ultimately transferred onto a media sheet, typically by employing yet another surface potential that attracts the toner off of the photoconductive surface (or an intermediate transfer surface) and onto the media sheet where it is ultimately fused.
The difference between the surface potential of the developer roller and the surface potential of undischarged portions of a photoconductive surface is sometimes referred to as a “white vector.” An optimal white vector achieves certain desirable characteristics, one of which is to provide a clean media sheet with little or no appreciable background toner in areas other than where printing is desired. The magnitude of the white vector needed to prevent background is a function of numerous factors, including developer material, environment, imaging device components, and age. Traditionally, imaging devices incorporating an electrophotography process operate with a white vector that is fixed, but large enough to overcome the factors that contribute to unwanted background.
Very large white vector values are not necessarily the most desirable solution because, although background will be limited, the density of deposited toner and detail of the resulting image may be adversely affected. Conversely, as white vector values fall, unwanted background may begin to appear. Determining an optimal WV that is somewhere between these extremes and that accounts for the aforementioned factors and varying operating conditions is a legitimate problem that is not solved by setting a fixed operating point.